The field of electronic devices, such as batteries, display elements (ECD, and the like), etc., continues to see increasing progress in terms of achieving high performance levels, size reduction, and thickness reduction. Accompanying this, the ionically conductive materials used in these devices have certainly also met high performance levels as well as various high-level requirements such as solidification, high reliability, high flexibility, high moldability, and moisture resistance, among others.
In this context, the following ionically conductive materials are already known:
(i) electrolyte solutions consisting of electrolyte dissolved in water, aqueous solvent, or organic solvent, PA0 (ii) inorganic solid electrolytes such a beta-alumina (beta-Al.sub.2 O.sub.3), lithium nitride (Li.sub.3 N), lithium iodide-alumina (LiI-Al.sub.2 O.sub.3), and rubidium/silver iodide, etc., and PA0 (iii) solid electrolytes consisting of the solution or dispersion of the salt of a Group I or Group II metal in a macromolecular resin matrix. PA0 (A) organopolysiloxane having at least two carboxyl group-containing hydrocarbon groups in each molecule and PA0 (B) polyoxyalkylene having at least two hydroxyl groups in each molecule, and, PA0 (C) metal ion from Group I or Group II of the Periodic Table, wherein said metal ion is dispersed in the aforesaid crosslinked copolymer. PA0 (A) organopolysiloxane having at least two carboxyl group-containing hydrocarbon groups in each molecule, PA0 (B) polyoxyalkylene having at least two hydroxyl groups in each molecule in a quantity such that the ratio between PA0 (C) salt of a metal from Group I or Group II of the Periodic Table at 0.5 to 20 weight parts for each 100 weight parts of the total quantity of components (A) plus (B).
However, with regard to the electrolyte solutions of category (i), due to the use in this material of a liquid such as water or organic solvent, the problem generally arises of leakage to the exterior of the electronic device, and this leakage is entirely capable of causing deterioration in device performance and damage to neighboring parts. In order to ameliorate these problems, ionically conductive material has also been prepared in paste or gel form by the admixture of polymer compounds into such electrolyte solutions. However, even these materials cannot completely eliminate the risk of leakage. The solid electrolyte materials of category (ii) can basically be used in high-reliability long-life electronic devices, and, in addition, these materials can fulfill the requirements of size and thickness reduction. However, the current situation prevailing in this category is that materials cannot be obtained which have a satisfactory conductivity at room temperature, and these materials have not reached the level of widespread practical application. The solid electrolytes of category (iii) not only essentially solve the leakage problem, as for category (ii) materials, but because they impart desirable properties associated with organic polymers, such as high flexibility and high moldability, they have received attention as materials which can meet the extensive demands of electronic devices as listed above. In this regard, the following properties are required of polymeric ionically conductive materials which are to be used in solid electrolytes:
(i) The quantity of dissolved electrolyte.(metal salt) must reach suitably high levels, and the capacity for ionic dissociation must be high.
(ii) The dissociated ions must be mobile in the polymer matrix. In the area of polymer structures which satisfy these conditions, crosslinked materials containing polyether segments, such as polyethylene oxide (PEO), etc., have been the subject of various investigations due to their relatively good conduction properties. However, limitations do arise on molecular mobility in simple crosslinked PEO homopolymer, and a satisfactory conductivity at room temperature is not obtained for this substance. In order to provide improvement in this area, investigations have been carried out on the synthesis of solid electrolytes which combine the PEO segment and the siloxane segment with its very high molecular mobility. For example, Japanese Patent Application Laid Open [Kokai] Numbers 60-216463 [216,463/85], 60-217263 [217,263/85], and 63-142061 [142,061/88] describe ionically conductive materials in which the lithium ion, etc., is dispersed in a crosslinked copolymer of PEO and siloxane bonded by the Si--O--C bond. However, since the Si--O--C bond is readily broken in the presence of water, the handling of such materials is very troublesome. In Solid State Ionics, Volume 15, p. 233 (1985), an ionically conductive material containing dispersed metal ion is disclosed in which polysiloxane with polyethylene glycol side chains is crosslinked and solidified using difunctional isocyanate. However, in this case, in order to obtain solidification up to a satisfactory strength level, the NCO group must be added in a suitable excess relative to the OH group. When this material is placed in a device such as a battery, the residual NCO groups pose the risk of reaction with the electrode material, thus raising problems for practical application. In addition, with regard to methods for the preparation of siloxane/PEO crosslinked materials, Japanese Patent Application Laid Open Number 62-209169 [209,169/87] lists crosslinking by means of exposure to radiation (electron beams, etc.) as well as through a platinum catalyst-mediated hydrosilylation reaction. This publication also discloses ionically conductive material prepared by dispersing metal ion in these materials. Analyzing these methods and considering first the use of platinum catalyst, the platinum remains in the system: not only can the obtained solid electrolyte not be used in display elements since it develops color, but the risk arises of a reduction in carrier ion (e. g., lithium ion ) mobility. In the case of radiation crosslinking, while the preceding problems can be eliminated, this method has not achieved practical application due to its very high associated equipment costs.
Thus, the solid electrolytes based on crosslinked substances from polysiloxane and another polymer material which have been introduced up to the present time suffer from problems with their properties or method of preparation, and are not satisfactory for practical application in electronic devices as described above.
The present inventors carried out extensive research in order to solve the problems under consideration, and discovered as a result that a material, consisting of the dispersion of particular metal ions in a particular crosslinked copolymer, has an excellent ionic conductivity while lacking the problems described above. The present invention was achieved based on this discovery.
The present invention takes as its object the introduction of an ionically conductive material which is ideally suited for use as a solid electrolyte in electronic devices such as batteries, display elements, etc., as well as the introduction of a method for the preparation of said ionically conductive material.